If it was around a lock or, some larger basin, something like that I could imagine it.
Some locks and junctions can have strong and very weird currents and inexplicably deep sections..

Other than that you could slip, knock yourself out.

Jump off a bridge into one ? - I bet that's a darwin award someone has bagged.

Many of them are deep enough to submerge shopping trolleys and bikes, fall on them and break your back or something.

I dunno about "so many" though?
I'd think a lot more die in natural streams or lakes, or steeply canalised watercourses or reservoirs.

I for one would not go in one as i think you'd immediately get a million nasty diseases.